Resistance
Electrical resistance is the opposition offered by a conductor to the flow of current around a circuit.
A good conductor has low resistance.
An insulator, or poor conductor, has high resistance.
Equation
Current, voltage and resistance are linked by an equation which comes from The rule that states that the current (I) flowing through a resistor (R) is directly proportional to the voltage (V) across the resistor, provided the temperature remains constant.:
voltage = current x resistance
V= IR
V = voltage in V
I = current in A
R = resistance in \(\Omega\)
| V = IR | V = I x R |
| \(I =\frac{\text{V}}{\text{R}}\) | I = V ÷ R |
| \(R =\frac{\text{V}}{\text{I}}\) | R = V ÷ I |
| V = IR |
| V = I x R |
| \(I =\frac{\text{V}}{\text{R}}\) |
| I = V ÷ R |
| \(R =\frac{\text{V}}{\text{I}}\) |
| R = V ÷ I |
Examples using Ohm’s Law
The current in a torch lamp is 0.3 A when the voltage across it is 3 V.
Calculate the resistance of the lamp.
Answer
V = 3 V
I = 0.3 A
R = \(\frac{\text{V}}{\text{I}}\)
R = \(\frac{\text{3}}{\text{0.3}}\)
R = 10 \(\Omega\)
The resistance of the torch lamp is 10 \(\Omega\)
Question
3 A flows through a 240 V lamp.
What is the resistance of the lamp?
R = \(\frac{\text{V}}{\text{I}}\)
V = 240 V
I = 3 A
I = \(\frac{\text{240}}{\text{3}}\)
R = 80 \(\Omega\)
The resistance of the lamp is 80 \(\Omega\)
Question
What is the voltage across a 20 \(\Omega\) resistor when a current of 2A flows through it?
V = IR
I = 2 A
R = 20 \(\Omega\)
V = \({2}\times{20}\)
V = 40 V
The voltage across the resistor is 40 V.
